132 CRITIQUES AND ADDRESSES. [vi. 



remains of reefs formed by coral polypes of different 

 kinds from those which exist now, enter largely into the 

 composition of the limestones of the Jurassic period ; 

 and still more widely different coral polypes have contri- 

 buted their quota to the vast thickness of the carboni- 

 ferous and Devonian strata. Then as regards the latter 

 group of rocks in America, the high authority already 

 quoted tells us : 



" The Upper Helderberg period is eminently the coral reef period 

 of the palaeozoic ages. Many of the rocks abound in coral, and are 

 as truly coral reefs as the modern reefs of the Pacific. The corals are 

 sometimes standing on the rocks in the position they had v/hen 

 growing : others are lying in fragments, as they were broken and 

 heaped by the waves ; and others were reduced to a compact limestone 

 by the finer trituration before consolidation into rock. This compact 

 variety is the most common kind among the coral reef rocks of the 

 present seas ; and it often contains but few distinct fossils, although 

 formed in water that abounded in life. At the fall of the Ohio, near 

 Louisville, there is a magnificent display of the old reef. Hemi- 

 spherical Favosites, five or six feet in diameter, lie there nearly 

 as perfect as when they were covered by their flower-like polypes ; 

 and besides these, there are various branching corals, and a profusion 

 of Gyatliopliyllia, or cup-corals." 1 



Thus, in all the great periods of the earth's history of 

 which we know anything, a part of the then living 

 matter has had the form of polypes, competent to sepa- 

 rate from the water of the sea the carbonate of lime 

 necessary for their own skeletons. Grain by grain, and 

 particle by particle, they have built up vast masses of 

 rock, the thickness of which is measured by hundreds of 

 feet, and their area by thousands of square miles. The 

 slow oscillations of the crust of the earth, producing great 

 changes in the distribution of land and water, have often 

 obliged the living matter of the coral- builders to shift 

 the locality of its operations ; and, by variation and 

 adaptation to these modifications of condition, its forma 



1 Dana, "Manual of Geology," p. 272. 



